CRS & Projections: How to choose a CRS

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# CRS & Projections: How to choose a CRS

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# CRS & Projections: How to choose a CRS

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This video gets into the technical details of finding a **suitable** projected CRS.

This is only a concern for geocomputations and GIS analysis - for straightforward mapping you can use our magic CRS - 4326.
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## There are no all-purpose CRS

> “There exist no all-purpose projections, all involve distortion when far from the center of the specified frame” (Bivand, Pebesma, and Gómez-Rubio 2013)

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## National Mapping Agencies to the rescue

If your data lie within country borders then you can take advantage of CRS specifically designed by *National Mapping Agencies*.

These agencies create nation-level CRS that allow precision...

- ... national infrastructe planning.

- ... cellular network coverage.

- ... coastal erosion monitoring.

- ... weather forecasting.

- ... maps for civilian, government and military uses.

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### Governments usually supply data in national CRS

GIS datasets obtained from government data portals are usually provided in the national CRS - for instance [geoportal.statistics.gov.uk](https://geoportal.statistics.gov.uk/datasets/local-authority-districts-may-2020-boundaries-uk-bfe).

We can then use `st_crs()` and `st_transform()` to project other datasets into the national CRS.

```r
national_gis_dataset <- read_sf("data/shapefiles")
your_own_data <- read_sf("data/points")

your_own_data %>% 
  st_transform(st_crs(national_gis_dataset))
```

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## RSTUDIO CODING SLIDE

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### Hunting for national CRS

There isn't a perfect process for finding national CRS and many countries just don't have one.

The first place I recommend searching is [EPSG.io](https://epsg.io)

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## When National CRS won't cut it

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## Oceanographic CRS

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There are Oceanographic CRS for entire oceans and small regions.

Small region oceanographic CRS are usually designed for oil exploration/extraction.

I recommend using [EPSG.io](https://epsg.io) to search for appropriage CRS for your data.]

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## Projection Wizard

[projectionwizard.org](https://projectionwizard.org/) is a free interactive tool for creating custom CRS.

... but to use it you need to decide what type of projected CRS you need.

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## Projected CRS Types

Source: [ihatecoordinatesystems.com/](https://ihatecoordinatesystems.com/)

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## RStudio Coding Slide